Spinning Superstuff

by Steve Miller

Scientists are studying remarkable materials from nature to discover ways to make them even better.

In a dark corner of your basement you may find one of nature's strongest materials. But you'll have to look closely because it's easy to overlook this barely visible, threadlike substance until you've bumped into it. Yes, you guessed it—it's spider silk!

The fine thread from which a spider frames its web and hangs in mid-air, called dragline silk, is stronger and far more elastic than a steel wire of the same diameter. If we had enough of this fiber we could make better parachutes, better artificial ligaments, and even better cables for suspension bridges. A rope of spider silk the diameter of a pencil might replace the 3-centimeter-diameter steel cable that is now used on an aircraft carrier to snag a hook on the underside of airplanes and help them land on the ship's short runway. As Randolph Lewis, a materials researcher at the University of Wyoming, Laramie, points out, “That's exactly what a spider's web is used for—to catch [an object, in this case] a fly moving at full speed.”

Scientists are trying to determine how a spider uses proteins similar to those in our fingernails to make a silk with exceptional strength and elasticity. Biophysicist Lynn Jelinski of Cornell University studies golden orbweaving spiders, Nephila clavipes, because they are large, easy to handle, and produce strong dragline silk. In order to prevent fights between the spiders, each has its own cage in Jelinski's lab. Her research team collects the silk the spiders spin and analyzes it to determine its chemical composition and structure. The researchers have found that the silk polymer consists of three types of protein segments, connected to each other. About one-third of the silk polymer is made up of two types of sub-microscopic crystals, which give the silk its strength. These crystals are connected to each other by extremely flexible strands, which give the silk its elasticity. The two characteristics combine to make silk that is both superstrong and stretchable.

Because this desirable spider silk cannot be obtained in large quantities like silkworm silk, scientists are working on ways to manufacture it in their labs. One way they have come up with is to implant spider silk genes into bacteria and yeast to produce proteins similar to those used by the spider. The silk protein solution is then forced through a small opening resembling a spider spinneret to make silk. Although the resulting silk thread is not exactly like the spider's, it does have similar properties.

Mimicking Nature

The spider silk project is just one example of biomimetics, the study of biological materials, processes, and structures in order to imitate or improve them. Modern analytical instruments and bioengineering techniques are expanding our ability to duplicate nature. The hinge of a fly's wing has to be very flexible and springy in order to move back and forth hundreds of times each second. It is made of a material called resilin, which has been described as the most perfect rubber ever found. Barnacles attach themselves to ships and whales with a glue so strong they are almost impossible to remove. Determining the chemical composition and structure of fly resilin and barnacle glue may make it possible for us to produce better rubber and adhesives.

In addition to studying the chemical structure and composition of natural materials, biomimeticists study how nature puts those materials together. A spider's web is a remarkable engineering achievement. Using a very small amount of material, the spider makes a large, durable structure. The trunks and limbs of trees are designed to hold very heavy loads while remaining flexible enough to survive strong winds and ice storms. To accomplish this, a tree uses one of nature's most common design elements—the linked hexagons of bees' honeycombs. Almost 90 percent of wood cells are arranged in this pattern to attain maximum strength with a minimum of weight. Engineers apply the honeycomb pattern to the internal structure of products that need to be both strong and lightweight, such as skis, airplane wings, and telescope mirrors.

By properly aligning flexible but strong fibers inside a hard polymer like epoxy, materials scientists have mimicked wood structure to make composite materials using fibers made of glass, graphite (carbon), or KevlarTM, a very strong type of nylon used to make bullet-resistant vests. Not surprisingly, the resulting composites are extremely strong, yet lightweight, which makes them useful in the construction of airplanes, boats, and sports equipment including rackets, skis, and snowboards. Pole-vault poles made of composite materials have allowed athletes to improve their vaulting height by almost 2 meters.

Researchers at the U.S. Air Force are studying other lightweight, strong natural materials to find ways to improve airplane and rocket components. The abalone, a type of shellfish, makes its shell by mixing calcium carbonate and proteins and layering the mixture to form a material as strong as the most advanced human-made composites. Studying the exoskeletons of insects may provide information on still another durable but light material that could be copied from nature.

Take a look around you. Mother Nature has some important lessons for us to learn. Who said copying wasn't allowed?

Spin, Spider, Spin!

by Steve Miller

A spider makes silk by combining protein molecules into long chains. A solution of proteins in water is produced in the spider's special glands and transferred to its spinneret through a narrow duct. While the molecules are in the duct they start to combine to form a polymer.

The spinneret has two parts, a valve and a spigot. The valve controls the thickness of the thread, which ranges in thickness from 0.1 to 8 micrometers. (A human hair is about 1 micrometer thick.) The spigot puts the silk where the spider wants it. The wet thread dries to form a strong, stretchy fiber. Some of the strands are thick, and they form the framework for the web. Others are thin and sticky, to trap food.

After the spider is finished with its web it frequently eats it to provide material for a new web.

Activity

What is the solute that a spider uses to create silk?[anno: The solute is protein.]

How does the spider use this solute to create silk?[anno: The spider mixes the solute with water and then pushes the solution through a narrow duct. The molecules combine in the duct to form a polymer. This polymer is ejected through the spinneret.]

What is the name for the field of science that studies biological materials, processes, and structures in order to recreate and use these materials?[anno: This field of science is called biomimetics.]

Think of a use, not mentioned in this article, for artificially created spider silk. Explain what the silk would be used to do and why this use would be beneficial. Write a few sentences to explain your answer.[anno: Answers will vary.]